Fixed bed reactors are often used in petroleum refining and chemicals for hydrocracking, hydroprocessing and reforming. These reactors are usually, but not necessarily cylindrical, and can have a diameter up to about 20 feet or larger, frequently with a height of about 20 ft to more than 100 ft, depending upon the application. Fixed bed reactors are filled with the catalyst particles, which are typically about 1 to 2 mm in size, but may be smaller or larger.
Typically, a reactor has multiple catalyst beds, two or more beds depending upon the application of the reactor. The usual feed to the reactor is an oil and may also include hydrogen. The purpose of the hydrogen depends on the operational function of the reactor, such as to hydrodesulfurize the feed, or to remove nitrogen, or to saturate aromatics, or to hydrocrack the feed.
In hydrocracking, processes having high temperatures and/or pressures and/or added hydrogen are used to crack large molecules into smaller more usable molecules. These reactors are used very often in petroleum industry for desulfurization, denitrogenation, aromatic saturation and hydrocracking.
There are many other reactions. But a common problem with the reactions in the fixed bed units is that most of the reactions are exothermic, i.e., as the feed flows through the reactor the temperature of the processed fluid increases. Further, these processes are constrained by radial temperature maldistributions. These temperature maldistributions cause significant safety and operations problems. For example, the safe operating windows in hydrocrackers is often constrained by temperature maldistribution in catalyst beds. In addition to safety, temperature maldistribution causes premature catalyst deactivation leading to much shorter run lengths. Also, temperature maldistribution often leads to poor product selectivity and higher hydrogen consumption.
Radial temperature maldistributions, i.e., temperature variations along the diameter of a catalyst bed may be quite significant, and at times the variations are as large as the temperature rise along the length of the bed. This lateral temperature maldistribution may be caused by many sources. There may be nonuniform flow, meaning higher vertical flow in one area of the bed than in other areas of the bed. When a fluid spends more time in the bed, the temperature can rise to a higher level, thus causing temperature gradients along cross-sectional portions of the unit. Such nonuniform flow can be caused by localized fouling, e.g., by an obstruction resulting from formation of polymer or coke in the catalyst bed. Nonuniform flow can also be caused by catalyst packing, which is not uniform and thus causes obstructions to occur more quickly in the more densely packed areas. There are many other reasons that may not be fully understood for causing radial temperature maldistribution. But, there is no doubt that there can be very large radial temperature maldistribution.
Temperature maldistribution in a catalyst bed normally gets propagated to the next downstream catalyst bed because of poor radial mixing in fixed beds. This propagated maldistribution gets further amplified because the reaction rates (heat generation rates) increase with temperature. This “snow balling” effect can lead to unsafe temperatures which can result in the above noted safety and operations problems, runaway reaction, catalyst deactivation, and catalyst fusion agglomeration.
Quench boxes, such as disclosed in U.S. Pat. No. 4,960,571, are often located between catalyst beds to control the reaction temperature and to provide radial mixing so that the temperature maldistributions from an upstream bed do not get propagated to a downstream bed. However, the quench boxes are expensive. Also, quench boxes are difficult to retrofit in existing reactors. U.S. Pat. No. 4,960,571 is hereby incorporated herein by reference.
It is apparent that there is a need for a simplified and relatively inexpensive technology to minimize temperature maldistribution in fixed bed catalytic reactors.